Hard on the heels of a decision to step up the frequency of Windows updates, Microsoft on Thursday announced it would give customers 17 months to stop using older versions of Internet Explorer (IE), including the most popular of them all, IE8.

The decision will further complicate enterprises' use of Microsoft's software, analysts said.

"This is huge," said Michael Silver of Gartner. "IE has been one of the biggest inhibitors if not the biggest inhibitor preventing organizations from moving to Windows 7 and Windows 8. I've spoken to organizations that said they'd have deployed Windows 8 if they didn't have to upgrade IE. This is another way Microsoft is trying to persuade, or force, organizations to keep current. For some organizations, like those in regulated industries, that's really difficult."

In a surprise announcement yesterday, the head of IE's marketing said that after Jan. 12, 2016, Microsoft would support IE9 only on Windows Vista, IE10 only on Windows Server 2012, and IE11 on Windows 7 and Windows 8.1.

IE7 and IE8 will drop off support completely, no matter what OS they run on.

The browsers will continue working, but Microsoft will halt technical support and stop serving security updates for the banned versions. Because of the large number of critical vulnerabilities Microsoft patches in its browser -- 84 in the last two months alone -- it will be extremely risky running an unsupported version.

Roger Capriotti, who leads IE marketing, cited a number of reasons for the change, including better security, less version fragmentation for Web app and site developers, and improved compatibility with third-party and Microsoft's own Web-based applications and services, such as Office 365.

"Running a modern browser is more important than ever for the fastest, most secure experience on the latest Web sites and services," Capriotti said in a long blog post Thursday.

The move was a repudiation of a decades-old support policy that promised to support an edition of IE for as long as the operating system(s) able to run it. Under the now-in-tatters policy, 2006's IE7 was to receive security updates until April 11, 2017, the call-it-quits date for Windows Vista. IE8, which launched in early 2009, and 2011's IE9 were to stay on the support list until Jan. 14, 2020, the retirement date for Windows 7.

Likewise, IE10, which launched in September 2012, was supposed to receive patches until April 9, 2023, the end date for Windows 8.

In other words, Microsoft just scratched off a year of support for IE7, four years for IE8 and IE9, and seven years for IE10. After Jan. 12, 2016, the only current browser -- Microsoft is sure to release others before then -- that will retain support on the dominant versions of Windows will be IE11.

What's striking about the support change is that Microsoft will abandon IE8, the most widely-used edition, in less than a year-and-a-half. According to data from metrics company Net Applications, IE8 was used by 37% of those running one form or another of Internet Explorer, more than the 29% share that the much newer IE11 controlled last month.

And IE8 use has been growing: In the last three months, its rate of growth has been four times that of IE11.

What was Microsoft thinking?

Al Hilwa, an analyst with research firm IDC, focused on the security angle. "We have a situation where the security consequences of using outdated software is like putting enterprises in a slowly-heating pot," Hilwa said in an email. "We are definitely reaching the boiling point in terms of hacker intrusions and exploitation. The problem is changing and software provisioning has to change with it."

But Silver and others saw more at work in Redmond than Capriotti let on.

"Microsoft suggests that users will have a better experience with newer versions of IE, and that's probably true, but this will also reduce Microsoft's support costs," said Silver.

Wes Miller of Directions on Microsoft concurred. "This wasn't a complete surprise. In the world of new efficiencies [at Microsoft], it didn't shock me that they did this. They're looking for ways to build better software faster," he said, referring to CEO Satya Nadella's oft-stated goal to change Microsoft's culture, including accelerating software release tempos and making development teams more accountable, productive and economical.

Starting in January 2016, Microsoft will only support the most-current browser for an operating system. For Windows 7, that means several versions of IE will have shortened lifecycles.